"This was a relatively small study, too small to show with confidence that the lower dose is as effective. It gives us preliminary, but far from definitive, evidence. Physicians should use their discretion, based on patient needs," he advised.

Zytiga (abiraterone acetate) costs more than $9,000 a month and patients typically remain on the drug for 12 to 18 months, researchers said. Even patients with the best health insurance can have co-pays of $1,000 to $3,000 a month.

This study found similar outcomes between 36 advanced prostate cancer patients who took 250 milligrams of the drug with a low-fat breakfast and 36 patients who took the standard dose of 1,000 milligrams on an empty stomach.

For both groups, the time to disease progression was about 14 months.

The study was to be presented Monday at an American Society of Clinical Oncology meeting. Findings presented at meetings are generally viewed as preliminary until they've been published in a peer-reviewed journal.

"Our results warrant consideration by doctors who care for prostate cancer patients, as well as payers," according to Szmulewitz.

He said the findings suggest that advanced prostate cancer patients who have difficulty affording the drug could, with close monitoring by their doctor, consider taking a smaller dose with a low-fat breakfast. That could lead to a per-patient savings of up to $7,500 each month.

"If we could reduce the cost of medication for this stage of the disease by a few thousand dollars each month simply by having patients take it with food, that would be significant," Szmulewitz said.

The researchers noted that taking the drug with a high-fat meal increased absorption of the drug even more. But high-fat meals raised levels of the drug more unpredictably than low-fat meals did, they said.

This year, more than 160,000 men in the United States will be diagnosed with prostate cancer. In 2017, the American Cancer Society estimates that nearly 27,000 men will die from the disease.